1. Field of the Invention
This invention relates to an imaging device comprising an army of imaging elements each comprising a photosensitive element for sensing light incident on the imaging element and for storing charge representing the incident light and a rectifying element, the imaging elements being arranged in rows and columns with each photosensitive element and the associated rectifying element being coupled in series between an associated first conductor and an associated second conductor for allowing charge stored at a selected imaging element to be read out by applying voltages to the second conductors to forward bias the rectifying element of the selected imaging element to cause a current representing the charge stored at the photosensitive element of the selected imaging element to flow through the associated first conductor.
2. Description of the Related Art
Such an imaging device is described in, for example, U.S. Pat. No. 4948966. FIG. 1 of the accompanying drawings shows an example of an imaging element or pixel 101 of known imaging device 100. Generally, the image sensor 100 will comprise a two-dimensional matrix of N rows and M columns of pixels with associated row 102 and column 103 conductors. However, in the interests of convenience, only one pixel is illustrated in FIG. 1.
The pixel 101 comprises a photosensitive diode PD and a switching diode SD coupled in series between the associated row conductor 102 and associated column conductor 103. In the example shown, the switching diode SD and photosensitive diode PD are arranged with the cathodes coupled together. A capacitor C is shown coupled across the photosensitive diode PD. The capacitor C may be the parasitic capacitance of the photosensitive diode PD or may be an additional capacitor added to increase the dynamic range of the image sensor 100.
Each column conductor 103 is coupled to a suitable charge sensitive amplifier 104 having a capacitive coupling between its negative input and its output.
In operation of such an imaging device light falling on the photosensitive diode PD of the pixel 101 during an integration period causes the capacitance C of the photosensitive diode PD to be discharged. When an appropriate voltage V.sub.R is applied to the associated row conductor 102 to forward bias the switching diode SD, a current starts to flow to recharge the photosensitive diode PD capacitance C enabling the charge stored at the photosensitive diode PD to be integrated by the charge sensitive amplifier 104, so allowing the light sensed by the photosensitive diode PD to be detected.
While the charge stored at a related pixel during a previous integration period is being read by causing the associated switching diode SD to become forward biased, the remaining unselected pixels in the same column still have their switching diodes reverse biased, and at least some of them may be sensing light.
In such an imaging device, vertical cross-talk may result from unwanted currents from unselected pixels flowing down the column or first conductor.
The result of this vertical cross-talk is that the integrated output from the charge sensitive amplifier 8 for the selected pixel may include contributions from all of the other pixels in that column. The main source of the unwanted column currents is "dynamic leakage" from the unselected pixels in that column. Thus, when a pixel is sensing light, the capacitance C of the photosensitive diode PD is being discharged by the photo-generation of charge carriers within the photosensitive diode PD, which in turn means that the voltage across the switching diode SD is changing. The changing voltage V.sub.x at the midpoint J between the two diodes causes a current: ##EQU1## to flow through the capacitance C.sub.D of the switching diode SD of the unselected pixel and hence down the associated column conductor 4, so causing vertical cross-talk. This could have the effect of removing information from an image because, for example, black text on a white background could appear white and so be lost in the background.